Renault has told Red Bull and Toro Rosso that its upgraded power unit will not be available before the United States GP, and there is a possibility that the teams will decide not to use it.
The French manufacturer had previously postponed the debut of the 'D-spec' until Russia.
While it will in theory be physically ready by then, it has yet to be fully proven on the dyno, and Renault does not want to risk failures.
"We will have some units that will be ready for race 15 [Sochi], but the fact is that we haven't got the full validation yet," Renault's Remi Taffin told Motorsport.com.
"The schedule is too tight, so we can't commit to race 15. We've discussed it, and we need to have some targets, the new target is race 16.
"It's just that we need to do the validation on this process, and any time we short cut this process, we've failed. So maybe it's better we finish races."
Some hope left
Taffin indicated that there was still a slim chance that the tests would be completed in time, but he admitted that was not expected to be the case: "We don't think so. That's why we discussed it with our customers.
"Now if you look at the plan and the timing we will be short on time, because the validation is not fully proved. So even if we've got the engine available I am pretty sure it will go to the race after."
This weekend Daniel Ricciardo, Daniil Kvyat and Carlos Sainz have all made double engine changes, and Max Verstappen has also had a change.
In theory all four drivers now have enough components to reach the end of the season with no penalties, assuming there are no failures - or at least they did until Ricciardo suffered a failure of his second new engine in FP3 today, which Christian Horner suggests is terminal.
If the teams concerned switch to the upgraded engines in Austin they will have to take grid penalties.
This opens up the intriguing possibility that Renault may have gone to the effort of ensuring that the revised engines are available in Austin, but the teams opt not to use them, and stick with the older spec.
The exception could be Ricciardo, who may now be forced to make an extra change anyway.
"It's possible, that's the way it is," said Taffin. "Whether they chose to use the engine or not, it's their choice. They will have the engines, they will have the numbers, they will have the green light whenever we have the validation process done."
The decision of the teams will clearly depend on how much of an improvement the new engine represents, in other words whether they would give up grid position in Austin for improved performance at the last three races.
"What we see on the dyno will be on track, for sure. Whether it's going to be a big step, or a big enough step, I don't know."